1. Field of the Invention
The present invention generally relates to image-forming apparatuses, and more particularly to an image-forming apparatus such as an electrophotographic copier, printer, or facsimile machine using a fixing unit including a power storage unit.
2. Description of the Related Art
Many image-forming apparatuses such as copiers, which form an image on a recording medium such as plain paper or an OHP sheet, employ electrophotography in terms of image formation speed, image quality, and cost. According to electrophotography, a toner image is formed on a recording medium, and the formed toner image is fixed on the recording medium by heat and pressure. At present, heat roller fixing is most commonly employed as a fixing method for safety reasons. According to heat roller fixing, a heating roller applying heat using a heating member such as a halogen heater and a pressure roller provided opposite the heating roller are pressed against each other so as to form a so-called nip part where the heating roller and the pressure roller are in press contact with each other. The recording medium on which the toner image has been transferred passes through the nip part to be heated and pressed. As a result, the toner image is fixed on the recording medium.
An increasing importance of environmental issues in recent years has caused the image-forming apparatuses such as copiers and printers to have more advanced energy-saving features. When energy saving in the image-forming apparatuses is considered, power saving in a fixing unit fixing toner on a recording medium cannot be ignored. According to a commonly employed method to reduce power consumption in the fixing unit during the stand-by state of the image-forming apparatus, the heating roller is maintained at a certain temperature slightly lower than a temperature for fixing during the stand-by state. When the fixing unit is used, the temperature of the heating roller is raised immediately to an enabling temperature at which the heating roller becomes usable. As a result, a user does not have to wait for the rise of the temperature of the heating roller. This method requires a certain amount of power to be supplied even when the fixing unit is not being used, thus consuming extra energy. It is believed that the energy consumption during the stand-by state corresponds to approximately 70 to 80% of the energy consumption of the components of the image-forming apparatuses.
Therefore, there is an increasing demand for further power saving by reducing energy consumption during the stand-by state. It is desired that no power be supplied when the fixing unit is not in use. However, if no energy is to be consumed during the stand-by state, it takes a few to more than ten minutes before the heating roller of the fixing unit rises to an enabling temperature of approximately 180° C. This is because a metal roller of iron or aluminum is mainly used as the heating roller, and thus the heating roller has a large thermal capacity. Such a wait period decreases user-friendliness. Accordingly, a heating method that consumes as little power as possible while realizing quick activation from a stand-by state is desired.
A period for the temperature of the heating roller to rise may be reduced by increasing input energy per unit time, that is, rated power. Many image-forming apparatuses performing high-speed printing, referred to as high-speed machines, support a supply voltage of 200 V. In Japan, however, commercial power for offices is normally 100 V and 15 A, and special modifications have to be made to the power supply-related facilities of the locations of installation of image-forming apparatuses to support the supply voltage of 200 V. Accordingly, supporting the supply voltage of 200 V is not very common as a solution to this issue.
That is, an attempt may be made to raise the temperature of the heating roller in a short period of time, but maximum input energy is determined by power supply as far as the commercial power supply of 100 V and 15 A is employed. In order to improve this situation, a voltage lower by a certain level is applied to the heating roller to delay the falling of the temperature of the fixing unit when the fixing unit enters a stand-by state (for instance, Japanese Laid-Open Patent Application No. 10-010913). Alternatively, a secondary battery as secondary power supply is charged during the stand-by state of the fixing unit, and when the fixing unit is started up, power is supplied from a primary power supply unit as well as the secondary battery or a primary battery so as to reduce startup time (for instance, Japanese Laid-Open Patent Application No. 10-282821).
According to the technique disclosed in JP10-010913, however, a voltage lower by a certain level is supplied to the fixing unit even during its stand-by state. Thus, it is considered that power saving is insufficient. Further, this technique does not focus mainly on making maximum power supply at the time of activating the fixing unit larger than power supply from a primary power supply unit. On the other hand, according to JP10-282821, when the fixing unit is started up, power is supplied thereto from the primary power supply unit and the primary or secondary battery. Generally, a lead storage battery, a nickel-cadmium battery, or a nickel-hydrogen battery may be used as the secondary battery. The characteristics of the secondary battery are such that its capacity is deteriorated and reduced by repeated charging and discharging and that its useful service life becomes shorter as a discharge current becomes larger. Further, there is also the phenomenon of capacity reduction due to the memory effect. Generally, even a secondary battery considered as having a longer useful service life against a large discharge current can only be charged and discharged approximately 500 to 1000 times. That is, if charging and discharging of such a secondary battery is repeated 20 times a day, the useful service life of the secondary battery comes to an end in a month or so. This increases the frequency of changing secondary batteries, thus taking time, causing trouble, and increasing running costs such as the cost of batteries to be changed. Further, a lead storage battery, which uses liquid sulfuric acid as an electrolyte, is not preferable for use in office equipment.
Further, there is also a problem in that a sudden current change or an in-rush current at the time of starting or stopping the supply of high power increases a load on a circuit for heating housed in the heating roller and causes an input current to flow through peripheral circuits, thus causing noise. Accordingly, it is not preferable to frequently switch on and off the supply of power from a large-capacity (high power level) secondary power supply. Further, supplying high power at a time may result in the oversupply of power, thus causing the temperature of the heating circuit to rise excessively.
As a fixing unit improved in the above-described points that can be more effective in power saving, reduce noise due to a sudden current change or an in-rush current at the time of starting or stopping the supply of high power, and prevent an excessive rise in temperature by reducing startup time, Japanese Laid-Open Patent Application No. 2002-184554, for instance, proposes a device in which: a chargeable and dischargeable capacitor is employed in a secondary power supply unit; a charger charges the capacitor of the secondary power supply unit with power supplied from a primary power supply unit; a switching unit performs switching between the charging of the secondary power supply unit and the supplying of power from the secondary power supply unit to a secondary heating element (heater); and the power supplied from the secondary power supply unit to the secondary heating element is controlled.
The device disclosed in JP2002-184554 includes a primary heater caused to generate heat by power supplied from a commercial power supply and the secondary heater caused to generate heat by power supplied from the secondary power supply unit including the capacitor so as to heat the heating roller of a fixing unit. The capacitor employed in the secondary power supply unit may be a chargeable and dischargeable electric double layer capacitor having a capacitance of approximately 2000 F sufficient for power supply for a few to tens of seconds. For instance, the power supply from the secondary power supply unit to the secondary heater is controlled by being switched on and off based on timing for shutting off the power supply.
The capacitor has the basic functions of causing the secondary heater to generate heat by power supplied from the capacitor, reducing startup time required for the heating roller to reach a predetermined temperature using the generated heat, and preventing temperature for fixing (fixing temperature) from lowering at the time of paper passing through the fixing unit (paper passing operation). In actual usage, it takes some time before the temperature of a fixing roller and a fixing belt, that is, fixing temperature, lowers, and therefore, it is possible to charge the capacitor during that period. However, if such paper passing operation is performed frequently at short time intervals with a small number of paper sheets passing through the fixing unit at a time, the secondary heater is caused to generate heat every time the operation is performed, thus reducing the stored energy of the capacitor. That is, in so-called energy savers having an extremely small fixing thermal capacity, there is a tendency for the fixing roller and the fixing belt to be deprived of heat by paper, toner, and a pressure member at the time of paper passing so that the fixing temperature suddenly drops, and the temperature of the fixing roller and the fixing belt starts to recover when the pressure member has warmed up. Accordingly, it is necessary for the secondary heater supplied with power from the capacitor to start to generate heat immediately after the start of paper passing operations. As a result, the stored energy of the capacitor is reduced by repeatedly performing paper passing operations with a small number of paper sheets passing through the fixing unit at a time.